1. Field of the Invention
The method which forms the subject matter of the invention concerns graphitisation of long carbon-containing products for the purpose of obtaining electrodes, long bars, tubes of circular or other section or any other long products made of graphite.
The invention also concerns a furnace for carrying out the method of the invention.
2. Discussion of the Background
Quite a large number of methods have been proposed for continuously graphitising carbon-containing materials in the form of solid pieces or in granular form. These processes have not generally given satisfactory results, because of the difficulty in finding refractory materials through which carbon-containing products, brought to near 3000.degree. C., can be moved, and also because of the difficulties encountered in heating these products to 3000.degree. C.
German patent No. 2 311 467 describes a method of continuously graphitising cylindrical carbon-containing products, which are passed vertically through the inside of a furnace.
In the method described in German patent No. 2 311 467, as shown in FIG. 1, the carbon-containing products (1) to be graphitised (which, in the case shown in the drawing, are bars of circular section arranged vertically end to end) enter the furnace (2) through a feed aperture formed by a guiding tube (3) and an annular graphite sleeve (4), which is connected to one pole of a source of electric current. The internal diameter of the sleeve is such that a large radial clearance is left between the sleeve and the column of carbon-containing products. Instead of passing directly between the sleeve and the carbon-containing products, the electric current passes through the coke particles (5) which fill the interior of the furnace (2) and are in contact both with the outer wall of the sleeve (4) and also with the column (1) of carbon-containing products, in a zone which starts below the sleeve, at the place where the caving-in slope of the coke, which leaves a funnel-shaped gap (6) below the sleeve, comes into contact with the column of carbon-containing products. The column is thus progressively heated, both through radiation and heat conduction from the coke particles, brought to a high temperature by the electric current passing through them and also through direct heating by the electric current which flows through the column below the funnel-shaped space (6).
At the same time as the column of carbon-containing products moves down into the furnace, it is accompanied by the layer of coke granules in the vicinity of the column. Thus coke granules are continuously fed into the top of the furnace at (8), to replace those which are extracted therefrom at the bottom by means of the rake (10) and discharge tray (9). Downstream of the tray, contact members (7) provide a connection outside the furnace between the column of carbon-containing products and the other pole of the source of electric current.
Electrical contact between the cylindrical lengths of carbon-containing products is provided for, at each end thereof, by a graphitised paste which hardens as the temperature rises.
A furnace of this type has the disadvantage that it needs to have two products passing through it: on the one hand the column of carbon-containing products, and on the other hand the coke granules used as a filler. It is necessary for the coke granules to pass through the furnace since, if they did not, they would be heated excessively in the upper zone of the furnace, where all the electric current emanating from the sleeve (4) passes through them. In addition, the granules are driven downwards both by the weight and by the friction, right along the column of carbon-containing products. It is therefore important for the granules to be extracted as regularly as possible from the bottom of the furnace. They are kept at an extremely high temperature through contact with the column of electrodes, despite the proximity of the water-cooled walls (11) of the furnace (1). The column of electrodes similarly leaves the furnace at an extremely high temperature, since all the electric current passes through it as far as the contact members (7).